In the past, the two following types of flap gates were known.
The first type had a cylinder device 1 installed under water which was used to raise and lower the front end side of a flap gate 2 which was supported on a base end side to rotate freely, using the base end side as a supporting point, as shown in FIG. 8. In an example of this type. Patent Reference 1 disclosed a flap gate which has a rod end of a tilted jack of a door body installed, for rotation with respect to the door body, in an attachment hole formed on the sea bed on the downstream side of the door body, so that the door body oscillates between the raised position and the lowered position.
The second type had a winch 3 installed on land which was used to raise and lower the front end of a flap gate 2 whose base end side was supported to rotate freely, using the base end side as a supporting point, as shown in FIG. 9. In an example of this type, Patent Reference 2 disclosed a flap gate in which the winch is driven by winding a rope upwards so as to raise a door body.
However, in the type disclosed in Patent Reference 1, the cylinder is set in one direction and is able only to operate so as to push the door body in a raising direction. In the type disclosed in Patent Reference 2, the winch is set in one direction and is able to only operate to raise the door body by winding the rope upwards. Therefore, there is a problem that if a force acts in a raising direction, the door body will oscillate, and as a result, waves will propagate in the inside of the harbor.    Patent Reference 1: Japanese Patent Application Kokai Publication No. H03-202503    Patent Reference 2: Japanese Utility Model Registration No. 3042896